Your problem is unrelated to how you have set border-radius. Fire up Chrome and hit Ctrl+Shift+j and inspect the element. Uncheck width and the border will have curved corners.
If there are contents within the div that has the curved corners, you have to set overflow: hidden because otherwise the child div's overflow can give the impression that the border-radius isn't working.
<!-- This will look like the border-radius isn't working-->
<div style="border: 1px solid black; border-radius: 10px;">
<div style="background: red;">
text!
</div>
</div>
<!-- but here the contents properly fit within the rounded div -->
<div style="border: 1px solid black; border-radius: 10px; overflow: hidden;">
<div style="background: red;">
text!
</div>
</div>
you may include bootstrap to your html file and you put it under the style file so if you do that bootstrap file will override the style file briefly like this
if you have parent element than your parent element must have overflow: hidden; property
because if your children content is getting oveflowed from parent border than your border will be visible .otherwise your borderradius is working but it is hide by your children content.